A. V. Burenin

Experimental studies of pulsed signal propagation from the shelf to deep sea

Results of an experimental study of low-frequency broadband pulsed signal propagation in a waveguide that includes the shelf zone, the continental slope, and the deep sea region are presented. Using phase-manipulated signals with central frequencies of 366 and 600 Hz, pulsed characteristics are measured at six points along the propagation track, the maximal distance from the source being 368 km. It is experimentally demonstrated that, in the presence of a negative sound velocity gradient in the near bottom layer on the shelf with a small bottom slope, the choice of the source position at the shelf bottom near the shoreline provides the formation of a continuous illumination zone in the deep sea near the USC axis and a stable pulsed characteristic with two main sound energy arrivals. The propagation velocity of the pulse that is last to arrive is identical (within the measurement error) to the velocity of sound on the USC axis at the point of reception. Possibilities for practical application of the results obtained from the experiment are discussed.

Application of Complex Acoustic Signals in the Long-Range Navigation of Underwater Objects

The creation of modern technical means of research and exploration of the ocean based on submersibles of different designations is a priority field of science and technology development. Creation of intellectual autonomous unmanned submersibles (AUS), which provide multidisciplinary wide-scale study of sea basins and the ocean bottom and new basic knowledge in oceanography, marine geology, biology, and power engineering, is a pressing issue [1]. Significant progress in the development of electric power systems promoted the creation of an AUS with an operating coverage range of hundreds of kilometers. It has become reasonable to equip them with systems of transmitting and receiving low-frequency acoustic signals with a propagation range not smaller than the operating range of the submersibles as means for remote control of the operation and mutual maneuvering during their group operation. Up to the present, the development of such systems has been hampered by a lack of technical solutions for the measurement of the time of signal propagation from a sound source to a receiver over a distance of hundreds of kilometers with the required accuracy. In this work, we present the results of experimental studies of the propagation of low-frequency broadband pulse signals in an underwater sound channel (USC) using state-of-the-art technologies for synchronizing the receiving and transmitting channels. This allowed us to acquire unique data on the stability of acoustic wave propagation velocity in the USC for developing the technology of measurement of the distance between the sound source and the receiver. The main objective of the experiment was to discriminate the part of the signal propagating closest to the USC axis and to measure the time of its propagation from the source to the receiver. In order to do this, the method developed by the authors for sounding the sea medium by complex phase-manipulated signals was used. This technology allows us to distinguish and identify the arrivals of acoustic energy over different beam trajectories [2, 3]. The experiment was carried out in August 2006 in the Sea of Japan. Figure 1 shows the schematic geometry and methodological peculiarities of the experiment. According to the commands of the receiving vessel, complex signals (M-sequences, 511 symbols, and 4 periods of the carrier frequency per symbol) were transmitted with an interval of 5 min at a frequency of 600 Hz by the sound source stationary located near the bottom at a depth of 40 m and a distance of 450 m from the coast. The receiving vessel was represented by a yacht, which was used to deploy the radio hydroacoustic buoy with a hydrophone. The yacht was maneuvering under sail in the zone of reliable recording of radio signals. The hydrophone was lowered approximately to the USC axis, whose location was found from the measurement of the vertical distribution of sound velocity using the hydrological profiler from the yacht (Fig. 2). Signals were received at six points of the path at a distance from the transmitter ranging from 55 km to 368 km (Table 1). The buoy with the hydrophone was drifting in this process, and the coordinates were measured from the yacht using a GPS navigator when the yacht was passing close to the buoy. The signal information and marks of the common time system were recorded on a PC. The systems of common time based on thermally stabilized generators were included into the transmitting and receiving systems and were started before the beginning of the experiment. This allowed us to measure the times of signal propagation between the corresponding points with an accuracy not less than 10 ‐8 s. Processing of the information consisted in the calculation of the cross-correlation function between the received signals and the mask of the transmitted signal preliminary recorded by the PC. From two to four arrivals of acoustic energy, which propagated over different beam trajectories, were recorded in the pulse characteristics obtained using this method. The latest and maximal (in amplitude) arrival was identified as the one that passed near the USC axis, because it propagated near the minimum of sound velocity along the shortest path

Study of how hydrological conditions affect the propagation of pseudorandom signals from the shelf in deep water

The paper examines how hydrological conditions affect manifestation of the acoustic “landslide” effect, which consists in focusing of acoustic energy in the near-bottom layer on the shelf and its transition to the axis of an underwater sound channel in deep water. We compare the results of experiments performed in the Sea of Japan in April 2014 and August 2006 on the same acoustic track, where the distance between corresponding points was more than 100 km. In April, the hydrological conditions in the shelf region of the track and in the upper layer of the deep-water part of the sea were characterized by the presence of a relatively weak (~0.35 s–1) negative vertical sound velocity gradient, whereas in August 2006, it was ~1.5 s–1. Experimental and numerical studies showed that the acoustic landslide effect also manifests itself under conditions of a weak negative sound velocity gradient, but the structure of the acoustic field trapped by the underwater sound channel has a more complex character with a time-expanded pulse characteristic. Nevertheless, its ordered, stable, and well-identified structure at all track points chosen for measurements make it possible to reliably create an efficient (with accuracies to hundredths of a percent) underwater navigation systems like GLONASS and GPS for the spring hydrology season.

Specificities of applying pseudorandom sound signals to measuring impulse responses on the shelf of the Sea of Japan

Solving the problems of underwater acoustic communication and navigation for controlling underwater objects greatly depends on a correct estimation of the hydrological and acoustical environment in the region. Analysis of the domestic and foreign experience in the field of navigational support of self-contained underwater devises shows that, to solve the problem, it is technically and economically advantageous to deploy a set of fixed sources of navigation signals in the region with a range of coverage that is at least not less than the size of the region of interest. At long distances and, especially, in a shallow-water sea, the key factors in solving the problem of navigation are correct determination of the efficient sound speed and the time of signal propagation for each path connecting sources and receivers.

A mobile measuring complex for studies in the field of acoustic navigation of remote submersibles

A measuring complex has been designed for experimental investigations of the effects that hydrophysical processes in a marine environment have on the quality of navigation using on-board equipment of submersibles in the process of signal reception from hydroacoustic beacons installed near the shoreline. The main systems and characteristics of the measuring complex are described.

Processing of acoustic signals and computer modeling instrumental and programming measuring complex for acoustic navigation investigations

This paper describes the main systems and results of testing a mobile instrumental programming complex developed for experiments on investigation of the influence of hydrophysical processes in the marine environment on the quality of solving the navigation problems using stationary acoustic beacons.

A hardware and software system for measuring the angular structure of the acoustic fields in acoustic tomography

A hardware and software system for measuring the angular structure of acoustic fields using the vector-phase methods for processing the complex M-sequence probing signals is described. The results of testing the system are given. A unique relationship between the change in the temperature condition on the stationary acoustic route and the times and angles of arrival of the acoustic energy is determined under controlled hydrological conditions. It is shown that, using vector receivers in tomographic schemes, it is possible to obtain an independent additional parameter of the pulse response of the diagnosed waveguide and increase thereby the efficiency of reconstruction of hydrophysical fields from acoustic probing data.

Peculiarities of the Formation of the Interference Structure of Scalar-Vector Acoustic Fields on the Shelf of the Sea of Japan

The results of experimental study of the spatial structure of the scalar-vector acoustic field formed during towing of a tone low-frequency emitter over the shelf of the Sea of Japan are discussed. The experiment was accomplished by towing the source of a tone signal with a frequency of 134 Hz at a depth of 20 m over various acoustic tracks at distances up to 10 km from an integrated receiving system consisting of a receiver of acoustic pressure and three orthogonal components of the acoustic pressure gradient. Special attention has been focused on study of the interference structure of the scalar and vector fields with provision of the technical reliability of the method and the results of the experiment under controlled hydrological conditions. We discuss the quantitative characteristics and peculiarities of the interference formation along tracks that differ in depth. The unique results of comparing the horizontal and vertical components of the fields are most interesting of all. They allowed us to reveal the existence of eddy structures in the acoustic field of the source over several tracks. We analyze the possibility of practical application of the results of our research.

Experimental Testing of High-Accuracy Underwater Range-Finding Technology

The aim of the study whose results are discussed in this paper was to conduct experimental and numerical research on improving a high-accuracy method, developed by the authors, of positioning underwater objects. For this, experimental testing of an improved range-finding technology was carried out, based on the inclusion into the measuring scheme of a block that can measure and monitor the sound velocity on the shelf area of a track connecting a source of navigation signals and an a receiver system imitator consisting of autonomous underwater apparatus. In addition, under natural conditions, we implemented a scenario in which range-finding data was provided to an autonomous underwater apparatus carrying out a mission in the water area at a distance of 300 km from the source of navigation signals using technical tools for controlling variability of the sound velocity on the shelf. A specific example was used to test the acoustic range-finding technology on a track with complex hydrological and bathymetric conditions, and an estimate was obtained for the accuracy of measuring distances during a 4 h drift of the autonomous underwater apparatus imitator.

Features of the propagation of pseudorandom pulse signals from the shelf to deep water in the presence of gyre formation on the acoustic track

The paper discusses the results of an experiment conducted in the Sea of Japan in March 2016 on an acoustic track 194 km long in winter hydrological conditions. The most complex case of propagation of pseudorandom pulse signals from the shelf to deep water in the presence of gyre formation on the acoustic track. An analysis of the experimentally obtained pulse characteristics show that at all points, a maximum, in terms of amplitude, first arrival of acoustic energy is recorded. This is evidence that at a given depth horizon, pulses that have passed the shortest distance through a near-surface sound channel at small angles close to zero are received first. The calculation method of mean sound velocity on the track, based on the satellite data of surface temperature monitoring, is proposed. We expect that the results obtained with this method can be successfully used for the purposes of acoustic range finding and navigation.